MRI has achieved prominence as a diagnostic imaging modality, and increasingly as an interventional imaging modality. The primary benefits of MRI over other imaging modalities, such as X-ray, include superior soft tissue imaging and avoiding patient exposure to ionizing radiation produced by X-rays. MRI's superior soft tissue imaging capabilities have offered great clinical benefit with respect to diagnostic imaging. Similarly, interventional procedures, which have traditionally used X-ray imaging for guidance, stand to benefit greatly from MRI's soft tissue imaging capabilities. In addition, the significant patient exposure to ionizing radiation associated with traditional X-ray guided interventional procedures is eliminated with MRI guidance.
MRI uses three fields to image patient anatomy: a large static magnetic field, a time-varying magnetic gradient field, and a radiofrequency (RF) electromagnetic field. The static magnetic field and time-varying magnetic gradient field work in concert to establish both proton alignment with the static magnetic field and also spatially dependent proton spin frequencies (resonant frequencies) within the patient. The RF field, applied at the resonance frequencies, disturbs the initial alignment, such that when the protons relax back to their initial alignment, the RF emitted from the relaxation event may be detected and processed to create an image.
Each of the three fields associated with MRI present safety risks to patients when a medical device is in close proximity to or in contact either externally or internally with patient tissue. One important safety risk is the heating that can result from an interaction between the RF field of the MRI scanner and the medical device (RF-induced heating), especially medical devices which have elongated conductive structures with tissue contacting electrodes, such as electrode wires in pacemaker and implantable cardioverter defibrillator (ICD) leads, guidewires, and catheters. Thus, as more patients are fitted with implantable medical devices, and as use of MRI diagnostic imaging continues to be prevalent and grow, the need for safe devices in the MRI environment increases.
A variety of MRI techniques are being developed as an alternative to X-ray imaging for guiding interventional procedures. For example, as a medical device is advanced through the patient's body during an interventional procedure, its progress may be tracked so that the device can be delivered properly to a target site. Once delivered to the target site, the device and patient tissue can be monitored to improve therapy delivery. Thus, tracking the position of medical devices is useful in interventional procedures. Exemplary interventional procedures include, for example, cardiac electrophysiology procedures including diagnostic procedures for diagnosing arrhythmias and ablation procedures such as atrial fibrillation ablation, ventricular tachycardia ablation, atrial flutter ablation, Wolfe Parkinson White Syndrome ablation, AV node ablation, SVT ablations and the like. Tracking the position of medical devices using MRI is also useful in oncological procedures such as breast, liver and prostate tumor ablations; and urological procedures such as uterine fibroid and enlarged prostate ablations.
In many of the foregoing cases, elongated or large surface area metallic structures may be present in interventional devices that are used during a procedure to deliver therapy or provide a diagnosis, implanted devices that are placed within the body to provide therapy or deliver a diagnosis, or the tools used to deploy or deliver the interventional or implanted device to the patient. Examples of interventional devices having metallic structures may include plaque excision devices, embolic traps, electrophysiology catheters, biopsy needles/tools, and stem cell delivery catheters. Examples of implanted devices having metallic structures may include cochlear implants, pacemakers, implantable cardioverter defibrillators, Insulin pumps, nerve stimulators, lead wires, prosthetic heart valves, hemostatic clips, and non-ferromagnetic stapedial implants. Finally, examples of deployment or delivery tools having metallic structures may include catheters, sheaths, introducers, guidewires, transseptal devices, and trochars.
As appreciated by those skilled in the art, these metallic structures may undergo heating during an MRI scanning process. This heating may be caused by numerous factors, including but not limited to eddy currents from MRI gradient switching, RF induced heating due to electromagnetic interactions between the metallic structure and the MRI transmit coil, and large current densities at metal/tissue interfaces (where heating may occur in both the metallic structure as well as the connected tissue). In all of these cases, it may be important to monitor the device temperature at a single or multiple points such that a safe level of device heating may be maintained.
In some of the foregoing cases, the interventional procedure may also include delivery of ablative therapy in the form of either heat, such as by radiofrequency delivery, laser delivery, microwave delivery, or highly focused ultrasound delivery, or freezing, such as by delivery of a cryogenic fluid. When the interventional procedure includes the delivery of ablative energy, it may be especially important to monitor the temperature of the therapy delivery point such that the therapy can be appropriately titrated. Thus, temperature monitoring is an important step for interventional procedures performed under MRI guidance.
Numerous methods and devices for measuring temperature are known and used in the medical device field. One exemplary device for measuring temperature is a thermocouple. Generally speaking, a thermocouple may be any conductor that generates a voltage when subjected to a thermal gradient. Thermocouples typically use two dissimilar metals to create a circuit in which the two legs generate different voltages that may be measured to determine a temperature value. Thermopile devices operate in a similar manner and are constructed by connecting a plurality of thermocouples in series or parallel. Another exemplary device for measuring temperature is a resistance thermometer or resistance temperature detector (RTD). This type of device operates by exploiting the predictable change in electrical resistance of materials with changing temperature, and is typically made of platinum. Yet another exemplary device for measuring temperature is a thermistor. Thermistors utilize a type of resistor that exhibits a varying resistance according to its temperature. Both positive and negative coefficient devices exist (PTC and NTC). As opposed to RTDs which are formed from pure metals, thermistors are generally formed from a ceramic or polymer.
One exemplary method of measuring temperature is known as radiation thermometry. Every object emits radiant energy, and the intensity of this radiation per unit area is a function of its temperature. In radiation thermometry, infrared thermometers are used to measure intensity of radiation. Radiation thermometry is also commonly referred to as optical pyrometry, radiometric temperature measurement, infrared thermometry, optical fiber thermometry, two color radiation thermometry, and infrared thermometry. Another exemplary method of measuring temperature is based upon the semiconductor absorption theory, and may be referred to as the method of “spectral analysis.” Spectral analysis uses gallium arsenide (GaAs) tipped fibers, and operates on the absorption/transmission properties of gallium arsenide crystal semiconductors. As the crystal temperature increases, its transmission spectrum shifts to a higher wavelength. The relationship between temperature and the wavelength at which the absorption shift takes place is predictable. The temperature value may be obtained by analyzing the absorption spectrum. Yet another method of measuring temperature is known as fluoroptic thermometry. When thermo-sensitive phosphor is stimulated with red light it emits light over a broad spectrum in the near infrared region. The time required for the fluorescence to decay is dependent upon the sensor's temperature. The measured decay time may be converted to temperature using a calibrated conversion table.
The foregoing known devices and methods for measuring temperature have numerous disadvantages and limitations. Thermocouples are inaccurate, susceptible to MRI-induced heating due to their metallic nature, and require conductive leads that can create a non-MRI safe condition. Resistance thermometers or RTDs require conductive leads that can create a non-MRI safe condition and are mechanically fragile. Thermistors also require conductive leads that can create a non-MRI safe condition and are mechanically fragile. With regard to radiation thermometry, radiation amplitude at body temperatures is small and requires large area detectors. Further, it is difficult to provide sufficient lensing at the tip of the catheter. Spectral analysis is expensive, potentially toxic in the body due to the use of gallium arsenide, and the fibers are difficult to manufacture. Fluoroptic thermometry is also an expensive and inaccurate process that requires calibration before each use. Further, it is difficult to localize the temperature measurement point, and process testing cannot be exposed to ambient light.
Current technologies for measuring temperature in an MRI environment are inadequate. Therefore, what is needed is a real-time temperature measurement system that is MRI safe, accurate, biocompatible, and cost effective.